This invention relates to an error signal detection circuit in a servo control device which produces a reference signal having a slope changing at a substantially constant gradient and obtains an error signal by detecting the value of the reference signal at a timing at which a detection signal from an object of control is obtained and, more particularly, to an error signal detection signal of this type capable of preventing change in the gain of servo loop when period of the reference signal has been changed.
As a prior art error signal detection method in a speed servo or rotational phase servo for a rotary drum or capstan in an R-DAT (rotary head type digital audio tape recorder), there is a method according to which an error signal is obtained by producing a reference signal having a slope which changes at a substantially constant gradient, detecting a rotation speed or rotational phase by an FG (frequency generator) or a PG (phase generator) provided on the rotary drum, and detecting the value of the reference signal at a timing at which the rotation speed or phase detetion signal is produced.
FIG. 2 shows a prior art speed servo control device utilizing this error signal detection method. In FIG. 2, a motor 10 is an object of control which, for example, is a drum motor in an R-DAT. An FG 12 is directly coupled to the rotaion shaft of the motor 10 and produces a detection signal at a period obtained by equally dividing one revolution of the motor 10. An integrator 14 is constructed by connecting a resistor 18 and a capacitor 20 to an operation amplifier 16. A constant voltage V1 is applied to the capacitor 20 through the resistor 18 to charge the capacitor 20 and the output of the capacitor 20 is changed to V1 at a slope of a constant gradient. By turning on of a reset switch 22, the capacitor 20 is discharged and its ouptut is reduced to zero.
A control device 25 turns on the reset switch 22 to reset the integrator 14 and causes an immediately preceding integrated value to be held in a sample hold circuit 24 at a timing of generation of a detection signal by the FG 12. The control device 25 causes the integrator 14 to repeat the integration operation by turning on the reset switch 22 and turning off the reset switch 22 after a lapse of a predetermined period of time in accordance with the count of a reference clock. Length of time from turning on of the reset switch 22 till reaching of the integrated value to V1/2 after turning off of the reset switch 22 is determined to be the period of the reference signal (FG reference period). This FG reference period is determined by the length of time during which the reset switch is ON.
The voltage value held in the sample hold circuit 24 is applied to a comparator 26 in which an error between the voltage value and the reference value V1/2 is detected. This error signal is applied to the motor 10 through a servo amplifier 28 so as to control the sample held value to become V1/2 and thereby control the speed of the motor 10 so that the period of the detection signal by the FG 12 (FG detection period) will coincide with the FG reference period.
FIG. 3 illustrates the operation of the circuit of FIG. 2. The reference characters in FIG. 2 designate the followings:
t0: time during which the reset switch 22 is ON PA0 .tau.: slope time (time during which slope 30 reaches V1 from 0) PA0 T0: FG reference period (=t0+.tau./2) PA0 t: FG detection period PA0 f: FG detection frequency (1/t) PA0 v: speed error voltage PA0 V1: maximum value of the speed error voltage
The reference signal is reset at rising of the FG detection signal, increases after a lapse of time t0 at the slope 30 of a constant gradient and reset again at rising of next FG detection signal. The time T0 (=t0+.tau./2) from resetting till reaching to V1/2 is determined as the FG reference period. The value v of immediately before resetting is held in the sample hold circuit 24 and this value v is used as the speed error voltage for controlling the speed of the motor 10.
If the FG detection period t is longer than the reference period T0 (i.e., the motor speed is slower than the regular speed), the value v becomes v&gt;V1/2 and the speed of the motor 10 increases whereas if the FG detection period t is shorter than the reference period T0 (i.e., the motor speed is faster than the regular speed), the value v becomes v&lt;V1/2 and the speed of the motor 10 decreases. Thus, the motor speed is conrolled to a speed corresponding to the reference period T0.
In the R-DAT, there are various playback modes such as a standard mode, a long time mode and a pre-recorded mode and also various operation modes such as a double speed playback and fast search. These modes can be realized by changing rotation speed of the rotary drum, capstan or reel table. In other words, each of these modes can be realized by changing the reference period T0 to a period corresponding to the rotation speed of the particular mode. In changing the reference period from T0 to T0' in the prior art device, as shown in FIG. 4, the time t0 is changed by T0'-T0 while retaining the gradient of the slope 30 unchanged.
In the prior art method in which, as described above, the reference period T0 is adjusted by the length of time t0 and the gradient of the slope 30 remains unchanged, the ratio of change of the speed error voltage v to the speed error varies which results in variation in the gain of servo loop which makes the servo system instable. For overcoming this problem and thereby realizing an optimum control in all speeds, a control is generally made for changing the gain in other portion of the circuit. This, however, requires a complicated circuit design. The same is the case with the phase servo.
It is, therefore, an object of the invention to provide an error signal detection circuit in a servo control device capable of preventing variation in the loop gain when the reference period is changed and thereby improving the stability of the servo system.